What causes "format of value must be k=v, received %q" in grafana/k6?

Ensure every comma-separated segment has exactly one '='; remove trailing/leading commas

format of value must be k=v, received %q

Error message

format of value must be k=v, received %q

What it means

A single comma-separated segment of K6_BROWSER_SCREENSHOTS_OUTPUT does not contain exactly one '=' (file_persister.go:52). The parser splits each segment on '=' and requires exactly two parts, so segments with zero '=' or with more than one '=' are rejected.

Source

Thrown at internal/js/modules/k6/browser/browser/file_persister.go:52

}

// parsePresignedURLEnvVar will parse a value such as:
// url=https://127.0.0.1/,basePath=/screenshots,header.1=a,header.2=b
// and return them.
//

func parsePresignedURLEnvVar(envVarValue string) (presignedURLConfig, error) {
	ss := strings.Split(envVarValue, ",")

	presignedURL := presignedURLConfig{
		headers: make(map[string]string),
	}
	for _, s := range ss {
		// The key value pair should be of the form key=value, so split
		// on '=' to retrieve the key and value separately.
		kv := strings.Split(s, "=")
		if len(kv) != 2 {
			return presignedURLConfig{}, fmt.Errorf("format of value must be k=v, received %q", s)
		}

		k := kv[0]
		v := kv[1]

		// A key with "header." means that the header name is encoded in the
		// key, separated by a ".". Split the header on "." to retrieve the
		// header name. The header value should be present in v from the previous
		// split.
		var hv, hk string
		if strings.Contains(k, "header.") {
			hv = v

			hh := strings.Split(k, ".")
			if len(hh) != 2 {
				return presignedURLConfig{}, fmt.Errorf("format of header must be header.k=v, received %q", s)
			}

View on GitHub (pinned to 93accf6570)

Solutions

Ensure every comma-separated segment has exactly one '='; remove trailing/leading commas
URL-encode '=' inside values as %3D and ',' as %2C
If the presigned URL cannot be encoded cleanly, put the signed query into a header.* entry instead of the url value
Validate the value with a one-liner before launching k6: echo "$V" | tr ',' '\n' | awk -F= 'NF!=2{print "bad segment: "$0; exit 1}'

Example fix

# before
K6_BROWSER_SCREENSHOTS_OUTPUT="url=https://h/?sig=abc,basePath=/s"
# after
K6_BROWSER_SCREENSHOTS_OUTPUT="url=https://h/?sig%3Dabc,basePath=/s"

Defensive patterns

Strategy: validation

Validate before calling

// Node/JS pre-check
function segmentsOk(v) {
  return v.split(',').every(s => (s.match(/=/g) || []).length === 1 && s.trim().length > 0);
}
if (!segmentsOk(process.env.K6_BROWSER_SCREENSHOTS_OUTPUT || '')) {
  throw new Error('K6_BROWSER_SCREENSHOTS_OUTPUT: every segment must be exactly k=v');
}

Prevention

Never paste presigned URLs with raw query strings into the env var - encode or move auth to headers
Avoid trailing commas and empty segments
Add the one-line awk/tr check to CI before k6 runs

When it happens

Trigger: K6_BROWSER_SCREENSHOTS_OUTPUT contains a trailing comma ('url=https://h/,', empty last segment), a bare token without '=' ('strict'), or a URL whose query string itself contains '=' ('url=https://h/?X-Amz-Signature=abc' splits into three parts).

Common situations: Pasting a presigned S3/Azure URL with signature query parameters into the env var; YAML/CI env files that strip quotes; values copied from documentation examples where the '=' inside the value went unencoded.

Related errors

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/2fda3f96838f9ca9. Report an issue: GitHub.